Preparations for the 1.0 release

* Less cluttered solution layout.
* Move examples to a repository of its own.
* Move Roslyn analyzer to a repository of its own.
* Enable central package management.
* Clean up csproj files.
* Add README file to NuGet packages.

src/Tests/Spectre.Console.Cli.Tests/Data/Settings/FooSettings.cs

@@ -0,0 +1,8 @@
+namespace Spectre.Console.Tests.Data;
+
+public class FooCommandSettings : CommandSettings
+{
+    [CommandArgument(0, "[QUX]")]
+    [Description("The qux value.")]
+    public string Qux { get; set; }
+}